Leveraging the Interplay of RAID and SSD for Lifetime Optimization of Flash-Based SSD RAID

Flash-based SSD RAID arrays are increasingly being deployed in data centers. Compared with HDD arrays, SSD arrays drastically enhance I/O performance and density, and reduce power, cooling, and rack space. Nevertheless, SSDs suffer aging issues. Especially, an SSD has limited endurance and needs to be replaced when it reaches to the end of its lifetime. Although prior studies have been conducted to address this disadvantage, effective techniques of RAID/SSD controllers are urgently needed to extend the lifetime of SSD arrays. In this article, we propose a novel RAID architecture, called FreeRAID, to leverage the interplay of RAID and SSD controllers to optimize the lifespan of flash-based SSD arrays. FreeRAID adds a new exploitable phase to the life cycle of flash blocks. In FreeRAID, flash space is separated into normal space and exploitable space, and they are used to serve normal data and approximate data, respectively. We design a dual-space management scheme for RAID controllers to intelligently allocate SSD spaces based on their aging status. Inside an SSD, we propose an adaptive flash translation layer for the SSD controller to maintain the reliability and space efficiency of flash memories. We implemented a prototype of FreeRAID based on an SSD array simulator. Our experiments show that FreeRAID can significantly increase the lifetime by up to 3.07 × compared with conventional SSD-based RAID arrays.